Refactor database layer to eliminate type switching and simplify architecture

- Replace type switching with clean repository pattern using sqlc-generated code
- Move ContentRepository interface and domain models to db package
- Create separate SQLiteRepository and PostgreSQLRepository implementations
- Remove unnecessary RepositoryAdapter and ContentClient interface duplication
- Update all clients (HTTP, Mock) to implement db.ContentRepository directly
- Add context.Context parameters to all repository methods (Go best practice)
- Eliminate duplicate domain models and type conversions
- Remove type aliases - use db package types directly throughout codebase
- Update engine, content managers, and API handlers to use repositories directly

Benefits:
- Zero runtime type switching overhead
- Single source of truth for domain models
- Clean package boundaries and separation of concerns
- Standard Go interface patterns with context support
- Easier testing with mockable repository interface
- Maintainable: adding new database types requires no changes to existing code

internal/db/repository.go

@@ -0,0 +1,103 @@
+package db
+
+import (
+	"context"
+	"database/sql"
+)
+
+// ContentRepository interface for accessing content data
+// This replaces the ContentClient interface from engine package
+type ContentRepository interface {
+	GetContent(ctx context.Context, siteID, contentID string) (*ContentItem, error)
+	GetBulkContent(ctx context.Context, siteID string, contentIDs []string) (map[string]ContentItem, error)
+	GetAllContent(ctx context.Context, siteID string) (map[string]ContentItem, error)
+	CreateContent(ctx context.Context, siteID, contentID, htmlContent, originalTemplate, lastEditedBy string) (*ContentItem, error)
+
+	// Collection operations
+	GetCollection(ctx context.Context, siteID, collectionID string) (*CollectionItem, error)
+	CreateCollection(ctx context.Context, siteID, collectionID, containerHTML, lastEditedBy string) (*CollectionItem, error)
+	GetCollectionItems(ctx context.Context, siteID, collectionID string) ([]CollectionItemWithTemplate, error)
+	GetCollectionTemplates(ctx context.Context, siteID, collectionID string) ([]CollectionTemplateItem, error)
+	CreateCollectionTemplate(ctx context.Context, siteID, collectionID, name, htmlTemplate string, isDefault bool) (*CollectionTemplateItem, error)
+	CreateCollectionItem(ctx context.Context, siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error)
+	CreateCollectionItemAtomic(ctx context.Context, siteID, collectionID string, templateID int, lastEditedBy string) (*CollectionItemWithTemplate, error)
+
+	// Transaction support
+	WithTransaction(ctx context.Context, fn func(ContentRepository) error) error
+}
+
+// ContentItem represents a piece of content from the database
+type ContentItem struct {
+	ID               string `json:"id"`
+	SiteID           string `json:"site_id"`
+	HTMLContent      string `json:"html_content"`
+	OriginalTemplate string `json:"original_template"`
+	UpdatedAt        string `json:"updated_at"`
+	LastEditedBy     string `json:"last_edited_by,omitempty"`
+}
+
+// ContentResponse represents the API response structure
+type ContentResponse struct {
+	Content []ContentItem `json:"content"`
+	Error   string        `json:"error,omitempty"`
+}
+
+// CollectionItem represents a collection container from the database
+type CollectionItem struct {
+	ID            string `json:"id"`
+	SiteID        string `json:"site_id"`
+	ContainerHTML string `json:"container_html"`
+	UpdatedAt     string `json:"updated_at"`
+	LastEditedBy  string `json:"last_edited_by,omitempty"`
+}
+
+// CollectionTemplateItem represents a collection template from the database
+type CollectionTemplateItem struct {
+	TemplateID   int    `json:"template_id"`
+	CollectionID string `json:"collection_id"`
+	SiteID       string `json:"site_id"`
+	Name         string `json:"name"`
+	HTMLTemplate string `json:"html_template"`
+	IsDefault    bool   `json:"is_default"`
+}
+
+// CollectionItemWithTemplate represents a collection item with its template information
+type CollectionItemWithTemplate struct {
+	ItemID       string `json:"item_id"`
+	CollectionID string `json:"collection_id"`
+	SiteID       string `json:"site_id"`
+	TemplateID   int    `json:"template_id"`
+	HTMLContent  string `json:"html_content"`
+	Position     int    `json:"position"`
+	UpdatedAt    string `json:"updated_at"`
+	LastEditedBy string `json:"last_edited_by"`
+
+	// Template information
+	TemplateName string `json:"template_name"`
+	HTMLTemplate string `json:"html_template"`
+	IsDefault    bool   `json:"is_default"`
+}
+
+// Helper function to convert sql.NullString to string
+func getStringFromNullString(ns sql.NullString) string {
+	if ns.Valid {
+		return ns.String
+	}
+	return ""
+}
+
+// ToNullString converts a string to sql.NullString
+func ToNullString(s string) sql.NullString {
+	if s == "" {
+		return sql.NullString{Valid: false}
+	}
+	return sql.NullString{String: s, Valid: true}
+}
+
+// FromNullString converts sql.NullString to string
+func FromNullString(ns sql.NullString) string {
+	if ns.Valid {
+		return ns.String
+	}
+	return ""
+}